The basic CMOS SRAM cell can be formed using cross-coupled CMOS inverters having 2-each N-channel and P-channel transistors. The cell is accessed by, typically, 2 N-channel control gates for a standard SRAM cell and 4 control gates for 2-port memory devices. Use of the P-channel transistors as the load devices for the SRAM cell results in the cell having favorable electrical characteristics.
The CMOS SRAMs may also use a four transistor cell design having resistive load devices in place of the P-channel transistors. This design is used in order to conserve physical layout area over the traditional six transistor cell design. This design also decreases chip costs. Two N-channel transistors are used to form a cross-coupled latch, while two additional N-channel transistors are used to provide access to the cell for reading and writing data. Two load devices are connected between the N-channel transistors in the latch and the power supply.
In the prior art, the resistive load devices are formed after formation of the N-channel transistors. After the transistors have been formed, a dielectric layer is deposited and contact openings are formed to the substrate. A second polycrystalline silicon layer is deposited and lightly doped N-type to achieve a resistivity in the range of 10.sup.6 to 10.sup.13 ohms/square. This blanket implant determines the load resistor value.
The second polycrystalline silicon layer also serves to provide interconnect between various portions of the integrated circuit. The second layer can be used for the V.sub.cc supply connected to the load resistors. This layer may also be used for local interconnect between various portions of the device. Thus, the interconnect portions of the second polycrystalline silicon layer must have a relatively low resistivity.
It is desirable to use a single polycrystalline silicon layer for both the resistive load devices and the interconnect or the V.sub.cc power supply. This approach is more economical than forming such regions from separate polycrystalline silicon layers, and also results in a relatively smoother chip surface. However, such an approach has an important drawback in that the resistivity of the resistive element region and the interconnect regions or the V.sub.cc power supply region are somewhat related. Historically, it has been difficult to form both very high resistance and very low resistance regions in a single polycrystalline silicon layer.
It would be desirable to provide a structure and method for fabricating high resistance and low resistance regions in a single polycrystalline silicon layer. It would be further desirable that a technique to form such regions is compatible with current technology and which adds a minimal amount of complexity to device process flows.